The commercial introduction of programmable unijunction transistors (PUT's) has had a significant impact in the design of electronic timers and oscillators for a wide range of applications. Specifically, the PUT allows a designer to program critical parameters such as standoff ratio, peak point voltage, peak point current, valley point current, and interbase resistance over ranges greater than those achievable by prior art true unijunction devices.
Although considered to represent a breakthrough in technology, available oscillator and timer designs employing PUT's have several shortcomings. The principal operating parameters specified by a designer are: (1) the peak point voltage (V.sub.p), (2) the peak point current (I.sub.p) and (3) the valley current (I.sub.v). As will be described in detail hereinbelow, the selection of certain discrete electrical components in prior art PUT timers can affect two or more of the operating parameters, rendering them interdependent upon one another. This interdependence necessities undesirable compomises in discrete component selection during the design optimization of prior art PUT timers. Additionally, prior art PUT timers often waste electrical energy by having an unduly large idling or standby currents as a result of the compromise selection of components.
It will be apparent from a reading of the specification that the present invention may be advantageously utilized within a wide range of applications and is not necessarily limited to automative tire monitoring applications. However, the invention is especially useful when applied to monitors of fluid pressure within the pneumatic tires of a relatively high speed "on road" motor vehicle, and will be described in connection therewith.
Since the invention of the pneumatic tire and its application to motor vehicles such as automobiles, trucks, and the like, many schemes have been proposed to monitor the fluid pressure in the tires during operation of the vehicles. Such systems facilitate maintaining optimum fluid pressure in pneumatic tires which, in turn, reduces the chances of catastrophic loss of vehicle control as well as extending tire life.
A problem in any tire pressure monitor is the transmission of fluid pressure information from the tire, i.e., a rotating body to the operator, i.e. a relatively fixed body. Most prior approaches to this problem fall within three general catagories. The first approach is the direct reading of fluid pressure in a tire in which sealed, rotating fittings or electrical slip rings are employed in the interface between the wheel and vehicle body. The second approach is the transmission of fluid pressure information through an inductive coupling involving two transducers, one on the wheel and the other on the vehicle body, which are in precise rotational alignment with one another. The third approach is the application of transmitters and receives which use the atmosphere to bridge the interface between the wheel and body. The first two approaches have several shortcomings. First, relatively expensive high precision components are needed in the slip rings, and inductive coupling transducers must remain in alignment at all times during operation of the vehicle. Additionally, both are prone to corrosion and mechanical wear at the point of interface as well as requiring relatively expensive body wiring from each wheel to a central location within the vehicle body.
Of the tire pressure monitors which have been commercially successful, most have followed the third approach, i.e., they have employed a transmitter affixed to each wheel which operates in conjunction with a receiver disposed within the vehicle body. These systems tend to be extremely expensive however, and require a breaking down of the tire from the wheel in order to service and/or replace the battery which powers the transmitter. Additionally, many prior transmitter-receiver approaches were of the near field type, requiring an antenna to be placed within one wave length of the transmitting antenna. This necessitated expensive and inconvenient body wiring from a central area on the vehicle body to a point adjacent each wheel. Finally, a few of the prior systems had the ability to verify that the system was operating correctly, i.e., had a self-check feature. Such prior systems typically comprised no more than a circuit continuity scheme in which a characteristic resistance was incorporated within the wheel mounted tire pressure switches and a window comparator function included in the receiver logic. These systems added more components and expense to the system without necessarily improving the reliability. Additionally, due to the dynamic vibration involved in normal vehicle operation, most prior systems have generally tended to deteriorate over a relatively short period of time, and those that did not suffer rapid deterioration were prohibitively costly for use with private passenger vehicles.
Finding a compromise solution for these problems has recently become more urgent in light of government and industry interest in the elimination of "spare" tires for cost and weight reasons and substituting "run flat" tires coupled with a tire pressure monitor.